White Dwarf Stars

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White Dwarf Stars

• The burnt-out cores of low mass, post-main
  sequence stars

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As cores contract, the density goes to
astronomical levels, matter acts in funny ways

• Gas in this room, the perfect gas law PVnRT.
  Pressure depends on both density and temperature
• Extremely dense, degenerate gas PVKn.
  Pressure depends only on density
• Demo

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The structure of a star a balance between
gravity and gas pressure
Self gravity
Gas pressure

• Technical term hydrostatic equilibrium

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Equations give radius of white dwarf as a
function of its mass
radius
mass

• What one might expect for how R depends on M

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What the solution really is for a white dwarf star
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Main features to note about white dwarf solution

• Note the size objects with masses like the sun,
  but radii like the Earth
• The size becomes smaller with increasing mass
• There is an upper limit (the Chandrasekhar mass)
  to the mass of a white dwarf

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Point to emphasize white dwarfs are real!
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Many white dwarfs have been cataloged
http//www.astronomy.villanova.edu/WDCatalog/index
.html
Note in particular 40 Eridani B, which is in The
sky now, and can be seen with small telescopes
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More information on 40 Eridani B
All Star Line Up.webarchive
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What happens to massive stars?
Cores are chemically differentiated And too
massive to be held up By electron degeneracy
force The cores collapse
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Core collapse of a massive star has two
consequences

• Massive explosion (1044 Joules)
• Production of a neutron star

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Formation of a neutron star from stellar core

• As core collapses, matter becomes compressed
• Electrons and protons forced together ep gt n
  nu (neutronization)
• Core of the becomes a neutron fluid
• Neutronization produces a burst of neutrinos
• Neutron fluid in core becomes degenerate and
  rigid

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The physics of a self-gravitating neutron blob
(neutron star)

• Radius versus mass relation for neutron star
• Notice size of neutron star
• Masses extend above Chandrasekhar limit

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Theoretical prediction of the existence of a
neutron star

• The remnant after the explosion of a massive star
• An object having the mass of the Sun (or more)
  but in an object with the diameter of Iowa City!
• An equivalent to the Chandrasekhar mass (largest
  possible mass of a neutron star)
• Do they exist?